Heat Shield

Our reentry simulations indicate we can keep the SSTT under 900 degrees C all the time. This is because the rocket is suborbital, small, and lightweight. Compared to shuttle reentry we will have much lower temperatures (about 900 C compared to 1650 C) and for much shorter time (about 2 min compared to 20 min).

Normally one of the hardest parts of making a reusable rocket is the Thermal Protection System (TPS). Because our rocket has a lower reentry temperature, we can use a simple industry standard metal alloy which lasts a long time. We don't need to go to exotic ceramic tiles that require a lot of repair every flight.

Heat Shield Materials

There are many interesting metal alloys that work at high temperatures also here. For our use the HAYNES 230 alloy looks good. In particular its ability to resist oxidization up to 2200 F / 1205 C for long duration and even hotter for short duration.

Another alloy is HASTELLOY-X which is good to 2200 Deg. F (1200 Deg. C). It seems Haynes 230 has a bit better oxidization resistance though.

But the best commercial alloy we know of is Haynes 214 which resists oxidization for long periods at 2300 F (1260 C) and short periods at 2400 F (1315 C).

Another option is to have a commercially available ceramic blanket. This could keep the heat from the main structure of the vehicle. The prices are between $1 and $10 per square foot.

Metalic Honeycomb TPS (Thermal Protection System)

If a high temperature superalloy is used to make a honeycomb core sandwich, you can get both the durability of metal and the light weight of honeycomb core. These can be about 1.25 to 2.5 lbs per sq foot.

For the X-33 work was done on metallic thermal-protection panels that are good to about 1,800 degrees Fahrenheit.

The ARMOR TPS is interesting. This is a metallic honeycomb sandwich panel made out of Inconel 617 which resists oxidization to about 1,800 degrees Fahrenheit (982 C). In a paper with interesting details on ARMOR they point out that the shuttle, which discards its main tank, is dense so it flies a hot reentry. A less dense reentry vehicle with lift could fly a reentry that kept below the melting temperatures of some alloys.

The following three papers are interesting;

An overview and assessment of metalic tiles.

A paper on heat shielding for orbital RLVs.

A report on metalic TPS.

Other interesting TPS links:

Metallic TPS solutions for increased heat loads "The cool inner background is made up of a porous material saturated with water, ensuring a temperature no higher than the boiling temperature of water. This system was tested in laboratory experiments and shows an increase of allowable heat flux for the PM1000 metallic skin from some 250 to almost 500 kW/m^2."
If the steam coming off from this was sent to the nose for transperation cooling, you could get two uses out of the same water you carried.
NASA has a database on thermal protection systems called TPSX.
A discussion of shuttle thermal protection - note 1.3 tiles per man-week for first shuttle

There are metals that will not melt or oxidize at very high temperatures. It is possible to make a metal foam that will insulate at very high temperatures.

A commercial coating called U2000 looks very interesting. It has been designed with reentry in mind. This coating with various ceramic matrix composites, carbon-carbon composites, and graphite substrates makes a very good TPS. It can weigh about 1.56 lbs/sq-ft.

The Space Shuttle used an aluminum frame but would have used titanium if they had a bit more money. Titanium would not have required tiles on the top of the orbiter.

Titanium was $3.44/lb in 1998, so the material cost would be under $3,000 for a 700 lb rocket. Worldwide titanium production was 240.7 million pounds in 1998. Companies like Boeing are big users. The USGS has lots of info on titanium.

An essay on the evolution of reentry technology.

The Wikipedia article on Atmospheric Reentry mentions a NACA Report 1381 about ballistic missile reentry from 1958 used to be online. When reentering the atmosphere the heat energy goes to either the air or the vehicle. In the case of a lightweight reentry vehicle the amount that goes into the vehicle is proportional to (skin-friction*surface-area) / (drag-coefficient * drag-reference-area). So the more the vehicle's drag, the less heat goes into the vehicle and the more goes into the air. A ball is actually the ideal shape.

There are Ceramic Fibers which might be interesting for making a high drag device.